Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog is usually expressed as "D-min", the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion (reduction sensitization) can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions. The chemicals used for preventing fog growth as a result of aging or storage are generally known as emulsion stabilizers.
The control of fog has been attempted by a variety of means. Thiosulfonates and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, and organic dichalcogenides, for example, the disulfide compounds described in U.S. Pat. Nos. 3,447,925; 2,440,110; 3,043,696; 3,057,725; 3,226,232; 3,397,986; 3,761,277; and 4,788,132 have been used to prevent the formation of fog in silver halide elements. Organic compounds having a polysulfur linkage comprised of three or more sulfur atoms, and organic compounds having a heterocyclic ring having at least two thioether linkages or at least one disulfur linkage, such as those described in U.S. Pat. No. 5,116,723, have also been discussed as suppressing fog and improving raw stock stability when used in combination with nitrogen-containing cyclic compounds.
Photographic emulsions that have a high silver chloride content are particularly prone to fog increase due to high temperature and humidity during storage. These changes may vary from layer to layer resulting in color imbalance and a loss of quality of the print material.
High chloride content color print paper also has an undesirable sensitivity to temperature changes during exposure. For example, when the temperature upon exposure rises due to heat from the exposing element during printing, the print density changes if the printing conditions are left at the initial set values. This may result in prints varying in density that were exposed at the normal temperature. Very often, an increase in temperature during exposure of the paper may result in a selective increase in speed in one layer, for instance the cyan layer, over another light sensitive layer such as the magenta layer. Such speed variation results in improper color balance of the color print, and requires the photofinisher to readjust his printing conditions in order to compensate for this density fluctuation. This results in a loss in operating efficiency.
Various methods have been described to reduce the temperature sensitivity of high silver chloride color paper materials. EP 0 367,227 (1988) discusses employing certain spectral sensitizing dyes in combination with mercapto azoles; and EP 0 325,235 describes using iron ion donating compounds in high chloride photographic elements to reduce their change in sensitivity due to exposure at elevated temperature. Other methods for improving heat stability are described in U.S. Pat. Nos. 5,443,947; 5,415,992; and 5,356,770.
Despite the vast amount of effort which has gone into methods to control fog in photographic elements there is a continuing need in the industry for practical and environmentally benign stabilizers which do not otherwise adversely affect the performance of the photographic element. There further remains a continuing need for an effective means for heat stabilizing high chloride emulsions against thermal changes.